2 answers

I edited Issac's answer because his div wasn't wide enough for the floats not to drop. The margin-left must be -0.5 x the width in this technique. You also see the likes of with:200% margin-left -100%. Also the images could be inline with a text-align centre on the container - no need to float them so long as the container is wide enough for both.
You will get a horizontal scrollbar to the right only on narrower screens. — Richard Grevers over 8 years ago